This invention relates to a process for the treatment of a mixed olefin feedstock, consisting essentially of a specifically defined vinylidene olefin component and a linear olefin component, to accomplish a selective separation between the two components and also a conversion of the vinylidene olefins to a valuable ester product.
The higher carbon number, e.g., C.sub.6 and higher, olefins are valuable commodity chemicals having a wide variety of end uses, including, for example, use in the preparation of olefin sulfonate surfactants, plasticizer alcohols, "detergent" alcohols and their derivatives such as alcohol ethoxylate surfactants, and synthetic lubricants. In commercial practice, these olefins are often obtained as mixtures of compounds having different molecular structures. For many applications of the olefins it is recognized to be important to separate such mixtures into components of like structure.
Of particular interest to the present invention is a separation of mixtures containing certain vinylidene olefins, e.g., compounds of the formula ##STR1## wherein R.sup.1 and R.sup.2 each represent an alkyl group. Conventional processes for the preparation of olefins from ethylene via chain growth on aluminum using Zeigler chemistry can result in mixtures of olefins containing substantial amounts of the vinylidene component. Vinylidene olefins are also found, although in lesser quantities, in olefin products synthesized from ethylene by other means, as well as olefins from other sources such as the thermal cracking of higher paraffins. It is recognized in the art (as described, for instance, in U.S. Pat. Nos. 3,557,236, and 3,686,250) to be beneficial in many instances to upgrade olefin mixtures to separate the vinylidene components of olefin products from their non-vinylidene components.
Because of similarities between the various olefin structural isomers in terms of boiling points, solubilities, and many other properties, it is generally not practical to separate olefins of like carbon number but different molecular structure using distillation, extraction, or other common separation techniques. This is particularly true of the higher carbon number olefins which are typically manufactured as mixtures of several carbon numbers. It has been proposed in the prior art (U.S. Pat. No. 4,511,753) to selectively remove vinylidene olefins from non-vinylidene olefins by contacting their mixtures with hydrogen sulfide or a hydrocarbyl mercaptan to convert only the vinylidene olefins to corresponding sulfides, and subsequently separating the non-vinylidene olefins from the sulfided mixture. Conversely, U.S. Pat. No. 3,291,853 discloses the selective conversion of linear alpha-olefins (olefins of formula I, wherein R.sup.1 is alkyl and R.sup.2 is hydrogen) from mixtures with vinylidene olefins upon contact with aluminum alkyls.
Also of substantial value in the chemical industries are higher carbon number, e.g., C.sub.9 and higher, carboxylic acid alkyl esters, which are known to find use, for example, in formulating medicines, ointments, cosmetics, and lubricating oils, as soaps, and as intermediates in the preparation of metal (e.g., aluminum or zinc) salts.
In one important respect, the present invention provides a process for a separation between certain vinylidene olefins and other olefinic compounds, which comprises a step for the contact of the mixture with a reactive agent selected from the group consisting of the alkyl esters of acrylic acid (for example, methyl acrylate), in the presence of an "ene" reaction catalyst. In the course of this step, the reactive agent converts the vinylidene olefin component to corresponding unsaturated alkyl esters. In general, the addition reaction of a vinylidene olefin (I) with an alkyl ester of acrylic acid (II) for preparation of a higher carbon number ester product (III) can be represented by the equation ##STR2## wherein R.sup.1 and R.sup.2 are as defined above for formula I, and R.sup.3 is alkyl. The specific higher ester product III in the equation is one of the isomers of such esters which result from the reaction. In the practice of the invention, this conversion to higher esters is highly selective for the vinylidene component. The reaction is found to be selective, in the sense that essentially all of a certain class of the vinylidene compounds can be converted, without significant conversion of the remainder of the compounds in the mixture. Prior art relevant to such a reaction step includes disclosures such as those of U.S. Pat. No. 3,783,136 which describes a process utilizing a catalyst of the form AlX.sub.3, where X is chlorine or bromine, to convert olefins, including linear alpha-olefins, to unsaturated mono- and di- carboxylic acid esters, and the similar process of U.S. Pat. No. 3,641,120 which utilizes a manganic carboxylic acid salt or oxide catalyst. In a later publication in J. Org. Chem., Vol. 39 (1974), No. 2, pp. 255-256, B. R. Snider reports that only 1,1-disubstituted (i.e., vinylidene) olefins are reactive in certain ene reactions, and that ene reaction products were not obtained with such olefins as 1-octene and cyclohexene. Snider does not report, however, any reaction of any vinylidene olefins in the presence of non-vinylidene olefins. The publication of A. Akermark and A. Ljungqvist, in J. Org. Chem., Vol. 43 (1978), No. 22, pp. 4387-4388, reports that Snider did not observe a conversion of 1-octene and like linear olefins because of an isomerization reaction which converted the linear olefin reactant to an internally-substituted reactant which then formed branched products. Using a modified catalyst, Akermark et al obtained a 40% conversion of 1-octene with methyl acrylate. Again, the reference fails to disclose a reaction involving a mixed, vinylidene and non-vinylidene olefins, and the high conversion of the linear alpha-olefin reported by Akermark et al as well as by the aforementioned patents is not suggestive of a separation process in which linear olefins are essentially unreactive. C. J. Albisetti et al (J. Amer. Chem. Soc., Vol. 78 (1956), pp. 2637-2641) also describe various reactions of either vinylidene or non-vinylidene olefins with methyl acrylate and the like.